Index funds strike me as a very bad idea. They reward unprofitable ventures just for being in some index. Buying into an index fund, therefore, is buying into a class of systemically overpriced assets. Yes, backtested, index fund investment does well---but that's because you're backtesting into an era before everyone was bleating "buy index ETFs!".

Between weird investment strategies, the length of the current boom, and the very low interest rates at the peak of the boom, this coming bust is going to be a massacre. I'm honestly not even sure whether the system as a whole will survive it.